1. Field of the Invention
This invention relates to the field of seals and gasket structures for blocking gaps between conductive structures against passage of electromagnetic interference (EMI), while also sealing the gap environmentally against airflow, noise, dust or the like. The invention provides a seal of this type wherein a conductive surface extends only part way around a resilient core, and the remainder of the surface is nonconductive, the respective conductive and nonconductive surfaces being defined by sheath portions which together enclose the resilient core.
2. Prior Art
Various forms of shielding to damp or block electromagnetic interference are known to be useful for sealing against walls or other fixed structures, mountings, movable cabinet closures and other aspects of electrical equipment. Such seals may be used to confine fields that affect the operation of nearby equipment, or that carry sensitive information signals. The seals are also useful to prevent electromagnetic fields originating in nearby unshielded equipment from inducing currents in shielded circuits. Radio equipment, computing equipment and the like generally include a grounded conductive enclosure at least for circuitry which operates at high frequencies, to block propagation of the fields generated therein. An enclosure of this type may or may not wholly enclose the circuits which generate or are sensitive to electromagnetic interference. For example, a simple grounded conductive sheet disposed adjacent the conductors of a circuit board can reduce electromagnetic interference that otherwise would couple high frequency signals among the components on the circuit board.
In a full or partial shielding enclosure which has a plurality of structural elements, connections are needed to couple the conductive components of the shielding enclosure electrically to one another and to ground. Where the cabinet walls form panels which are bolted together, or otherwise do not define continuously conductive structures the panels must be coupled across any gaps. Therefore, one use of conductive seals is to bridge electrically across conductive structural elements such as cabinet panels and doors. The shield simply fills any gap between conductive panels, thus blocking the path through which interference could propagate. For high frequency signals a small gap is sufficient to enable leakage. The conductive seals are made resilient such that they expand as necessary to fill the gap and to accommodate any irregularities in the gap.
These seals are also useful in that they can environmentally close gaps between integrally continuous conductive panels of an enclosure. The seals thus exclude dust and noise, confine cooling air, etc. For maximum performance in environmental sealing the seal must closely engage the panels or other structures at a gap. This requirement is met in a seal that includes a resilient core for urging sealing surfaces against the panels, provided the sealing surfaces are apt for forming a close fitting seal over a sufficient area of contact. However, materials which are optimal for EMI shielding may be less than optimal for environmental shielding, and vice versa. In addition, the cost of an EMI shielding material is normally substantially higher than that of environmental sealing materials.
U.S. Pat. No. 4,857,668--Buonanno discloses a seal or gasket of a type having a continuously molded resilient foam core enclosed by a conductive sheath. The sheath can comprise a woven or non-woven resinous fabric such as nylon, plated with a elemental metal, a metal alloy or a combination of metallic platings over one another. The foam core expands in the molding process to produce a resilient body upon curing, the core material engaging the conductive fabric as the foam expands and cures. This seal provides a good electrical seal due to the conductivity of the sheath, and a good environmental seal against dust and the like, particularly because the sheath is flexible and the foam core urges the sheath to conform to irregularities of the sealed gap.
It is not always desirable to conductively bridge across all the conductive panels or sections of panels which may converge at a seal. Certain panels of subassemblies (e.g., inner conductive enclosures) may need to be electrically isolated from other panels (e.g., outer conductive enclosures). Similarly, the points at which electrical connections are made between subassemblies is a matter of concern in that the subassemblies should be referenced to a common ground. If connections are such as to provide a plurality of ground connections, relatively small potential differences between the respective ground connections result in large currents due to the low electrical resistance of the conductive paths along so-called ground loops. To improve shielding, electrical connections must be carefully planned. According to known techniques, combinations of conductive and nonconductive seals have required the use of separate seals for the conductive and nonconductive passages, respectively. A particular situation requiring attention to conductive and nonconductive aspects of sealing is the input/output plane at which signal and/or power connections traverse the wall of a conductive electronic enclosure, an application which can be termed an "I-O shield".
Various metals and combinations of metals can be used to form a conductive sheath in an EMI seal. The particular choice affects the effectiveness of the seal. If the surface finish is corrosion prone, the electrical surface contact resistance increases over time as the exposed metal surface becomes oxidized. It is possible to use a noble metal to decrease corrosion problems, e.g., gold, platinum, silver, nickel, etc. Similarly, a metal mesh or plated fabric having a relatively more corrosion prone metal can be plated thinly with a noble metal to render the surface less prone to corrosion. Another possibility is to employ a chemical coating, such as a chromate conversion coating on aluminum, to eliminate oxidation. The need to provide a corrosion resistant surface on the seal as necessary to keep the electrical contact resistance low in any event increases the expense of the seal.
In the seal according to Buonanno '668, the conductive sheath extends fully around the cross section of the core. This arrangement provides two layers of conductive material across the sealed gap (i.e., the layers on the two opposite sides of the seal). The seal performs well for EMI blockage and environmental sealing, but the cost is relatively high.
EMI seals frequently are used to seal movable parts such as cabinet doors. Over time the contact surfaces of seals having thin platings of noble metal or chemical treatments of the seal surface can wear through. One result is increased contact resistance due to oxidation of the exposed conductive material of the seal and/or the sealed surfaces. Another result is accelerated corrosion of the seal and/or the sealed surfaces generally, due to the galvanic action of the dissimilar metals employed, particularly in the presence of moisture. Corrosion makes the sealed surfaces more abrasive, and over time the electrical contact resistance of the seal increases. In U.S. Pat. No. 5,045,635--Kaplo et al, a combination EMI and environmental seal is provided. A sheathed foamed core as in Buonanno is coated further with a conductive coating comprising conductive particles suspended in a non-reactive and nonconductive resin, the particles being provided in sufficient concentration that conductive paths are provided through the resin due to contact between the conductive particles. The particles can be conductive carbon black and the binder a urethane paint. This further coating provides improved abrasion resistance, environmental sealing and stable electrical contact resistance. The coating is applied at least on that portion of the seal which engages an opposed cabinet wall, door or other sealed surface, i.e., the area in which the seal is subjected to abrasion against the sealed surface.
U.S. Pat. No. 3,140,342--Ehrreich et al discloses a strip seal arrangement having conductive and nonconductive portions which are formed such that the conductive portion extends along one side of an elongated seal and the nonconductive portion extends along an opposite side. The conductive and nonconductive portions are each defined by coextensive elongated bodies of resin material forming the seal, the difference being that only the conductive one has metal particles therein. In this form of seal, the metal particles make the conductive portion less compressible and the nonconductive portion provides the needed resilience. Ehrreich discloses the possibility that one or the other of the portions can be a foamed elastomer but does not attempt to sheath a foamed elastomer with a conductive sheath.
U.S. Pat. No. 3,555,168--Frykberg teaches a partially sheathed foam material in a planar or sheet form of seal which resides on the inner side of a cabinet door. The seal is a laminate of adhesive, foam, adhesive and metal foil. The foam is adhered to the cabinet door via the adhesive, and when the door is closed the foam urges the foil glued onto the opposite side of the foam into contact with cabinet walls against which the door is engaged.
U.S. Pat. Nos. 3,089,915 and 3,624,267, both to Plummer, are examples of shielding configurations which define sheets to be wrapped around cables and the like. The sheets have a conductive layer on one side, normally the side to be directed toward the cable, and a nonconductive layer on the other side, for environmental protection of the cable and shield. In order to completely encircle the cable, the ends of the sheet are overlapped when the sheet is wrapped around the cable. Moreover, the conductive layer is wrapped around the edge of the nonconductive layer in order to make an electrical connection between the edges in the area of the overlap.
The present invention concerns a compressible seal for residing in a gap. The conductive and nonconductive surfaces of the seal define an enclosure for the compressible core material. The nonconductive surfaces form a good environmental seal and the conductive layer functions as in Buonanno '668 to define an shielding partition bridging the gap for blocking propagation of electromagnetic energy. The seal is inexpensive and effective, and provides stable performance over time.